Phase-shifting mask with multiple phase-shift regions

ABSTRACT

Transmitting portions of a phase-shifting mask include plural first transmitting areas periodically arranged along a first direction and a second direction and a second transmitting area provided in an area surrounded with adjacent four first transmitting areas among the plural first transmitting areas. The first transmitting areas are formed by recessing a board so that a phase difference of substantially 180 degrees in exposure light can be caused between adjacent first transmitting areas. A phase difference of substantially 90 degrees in the exposure light is caused between the second transmitting area and the surrounding first transmitting areas. Thus, isolated patterns arranged at high density can be formed correspondingly to the first transmitting areas and the second transmitting area.

This application is a Divisional of application Ser. No. 09/107,443filed Jun. 30, 1998 now U.S. Pat. No. 6,114,095.

BACKGROUND OF THE INVENTION

The present invention relates to a phase-shifting mask and a method ofmanufacturing an electronic device using the mask. More particularly, itrelates to formation of contact holes arranged at a high density inmanufacturing a semiconductor integrated circuit device.

A dynamic random access memory (hereinafter referred to as the DRAM)functioning as a technology driver for a semiconductor integratedcircuit device has played a significant role in promoting refinement andhigh integration of elements in a semiconductor integrated circuitdevice. Now, mass production of a DRAM of 64 Mbits designed inaccordance with a 0.25 μm rule has been realized. Furthermore,production of a DRAM of 1 Gbit designed in accordance with a 0.18 μmrule has been expected.

In a DRAM of 1 Gbit, it is necessary to form each memory cell in an areaof approximately 0.3 μm², and hence, there is a demand for a cell pitchof 0.4 μm. A memory cell of an 8F² structure used in a memory includesone storage node contact in each cell. Therefore, it is necessary toadopt a technique to form a contact hole with a diameter ofapproximately 0.2 μm with a pitch of 0.4 μm or less.

As a technique to improve resolution in patterning contact holes throughexposure, an attenuated phase-shifting mask is proposed. In thistechnique, a semi-transparent film is formed in a field region of aphotomask, thereby shifting the phase of the field region by 180° fromthe phase of a transmitting hole region. Thus, interference is causedbetween transmitted light of these regions, so as to sharply define apattern edge. By adopting this technique, an isolated contact hole witha diameter of approximately 0.2 μm can be formed. In this technique,however, it is difficult to separate contact holes from one another whenthe pitch therebetween is smaller than approximately 0.5 μm.

Therefore, use of an alternating phase-shifting mask having a higherdegree of interference has been proposed for formation of contact holeswith a smaller pitch (Japanese Laid-Open Patent Publication No.62-50811). An alternating phase-shifting mask is a photomask providedwith a phase shifter to every other transmitting areas for definingcontact holes to be formed, so that a phase difference of 180 degreescan be caused between light transmitted by adjacent patterns. Such aphase shifter is generally formed by etching a photomask plate. Byadopting this technique, a contact hole with a pitch of approximately0.3 μm can be formed.

The aforementioned phase-shifting mask is effective in forming a contacthole array where contact holes are periodically two-dimensionallyarranged in an array of rows and columns. However, it is difficult toapply the phase shifting mask to contact holes to be formed in positionsshifted from the regular matrix positions.

When a DRAM memory cell with an area of 0.3 μm² or smaller is to beformed, it is preferred that metal plugs are formed in a storage nodecontact hole and a bit line contact hole as is disclosed in, forexample, IEDM '96 Technical Digest, p. 593. When these two types ofcontact holes are to be simultaneously formed, the storage node contactholes are regularly arranged in an array of rows and columns, but thebit line contact holes are disposed in positions shifted by a ½ pitch.Accordingly, it is difficult to form these contact holes at the sametime by using the alternating phase-shifting mask. Therefore, after thestorage node contact holes are formed by using the alternatingphase-shifting mask, the bit line contact holes are required to beseparately formed by using an attenuated phase-shifting mask or thelike. As a result, discrepancy in locations of these contact holes,i.e., relative placement errors, cannot be avoided, which obstructs thedevelopment of refined memory cells.

SUMMARY OF THE INVENTION

The phase-shifting mask of this invention comprises a transparentphotomask plate, an opaque portion formed in the transparent photomaskplate and transmitting portions formed in the transparent photomaskplate, and the transmitting portions include plural first transmittingareas periodically arranged along a first direction and a seconddirection different from the first direction; and at least one secondtransmitting area provided in an area surrounded with adjacent fourfirst transmitting areas among the plural first transmitting areas, aphase difference of substantially 180 degrees in exposure light iscaused between adjacent two first transmitting areas among the pluralfirst transmitting areas, and a phase difference of substantially 90degrees in the exposure light is caused between the second transmittingarea and the four first transmitting areas surrounding the secondtransmitting area.

Preferably, a surface of at least a part of the plural firsttransmitting areas is positioned at a different level from a mainsurface of the transparent photomask plate.

Preferably, a surface of at least a part of the plural firsttransmitting areas is recessed to be lower than the main surface of thetransparent photomask plate.

Preferably, a surface of the second transmitting area is positioned at adifferent level from surfaces of the plural first transmitting areas.

Preferably, the surface of the second transmitting area is positioned atthe same level as the main surface of the transparent photomask plate.

Preferably, the phase difference caused between adjacent two firsttransmitting areas among the plural first transmitting areas is in arange between 160 degrees and 200 degrees.

Preferably, the phase difference caused between the second transmittingarea and the four first transmitting areas is in a range between 70degrees and 110 degrees.

In one aspect of the phase-shifting mask, the first direction can beperpendicular to the second direction.

In another aspect of the phase-shifting mask, each of the plural firsttransmitting areas has a shape for forming a hole.

In still another aspect of the phase-shifting mask, the secondtransmitting area has a dimension larger than a dimension of each of thefirst transmitting areas.

In still another aspect of the phase-shifting mask, a pitch of the firsttransmitting areas measured along the first direction is approximatelytwice as large as a pitch of the first transmitting areas measured alongthe second direction.

Preferably, a dimension of each of the first transmitting areas measuredalong the second direction is smaller than a dimension thereof measuredalong the first direction.

In the method of manufacturing an electronic device of this invention, aphase-shifting mask including a transparent photomask plate, an opaqueportion formed in the transparent photomask plate and transmittingportions formed in the transparent photomask plate is used, thetransmitting portions of the phase-shifting mask includes plural firsttransmitting areas periodically arranged along a first direction and asecond direction different from the first direction; and at least onesecond transmitting area provided in an area surrounded with adjacentfour first transmitting areas among the plural first transmitting areas,a phase difference of substantially 180 degrees in exposure light iscaused between adjacent two first transmitting areas among the pluralfirst transmitting areas, and a phase difference of substantially 90degrees in the exposure light is caused between the second transmittingareas and the four first transmitting areas surrounding the secondtransmitting area, and the method comprises the steps of forming aresist layer on a film used for forming a part of the electronic device;irradiating the resist layer with the exposure light through thephase-shifting mask; developing the resist layer; and patterning thefilm partially covered with the resist layer.

In the method of manufacturing an electronic device, the electronicdevice can be a semiconductor integrated circuit device, and the firsttransmitting areas and the second transmitting area of thephase-shifting mask can define openings formed in the film.

In the method of manufacturing an electronic device, the semiconductorintegrated circuit device can be a dynamic random memory, each of thefirst transmitting areas can define a storage node contact hole forconnecting a memory cell and a storage part, and the second transmittingarea can define a bit line contact hole for connecting the memory celland a bit line.

In the method of manufacturing an electronic device, off-axisillumination can be adopted in the step of irradiating the resist layerwith the exposure light through the phase-shifting mask.

Another phase-shifting mask of this invention comprises a transparentphotomask plate, an opaque portion formed in the transparent photomaskplate and transmitting portions formed in the transparent photomaskplate, and the transmitting portions include at least four firsttransmitting areas; and at least one second transmitting area providedin an area surrounded with adjacent four first transmitting areas amongthe first transmitting areas, a phase difference of substantially 180degrees in exposure light is caused between adjacent two firsttransmitting areas among the adjacent four first transmitting areas, anda phase difference of substantially 90 degrees in the exposure light iscaused between the second transmitting area and the four firsttransmitting areas surrounding the second transmitting area, wherebypatterns corresponding to the first transmitting areas and the secondtransmitting area are separately formed.

In another method of manufacturing an electronic device of thisinvention, a phase-shifting mask including a transparent photomaskplate, an opaque portion formed in the transparent photomask plate andtransmitting portions formed in the transparent photomask plate is used,the transmitting portions of the phase-shifting mask includes at leastfour first transmitting areas; and at least one second transmitting areaprovided in an area surrounded with adjacent four first transmittingareas among the first transmitting areas, a phase difference ofsubstantially 180 degrees in exposure light is caused between adjacenttwo first transmitting areas among the adjacent four first transmittingareas, and a phase difference of substantially 90 degrees in theexposure light is caused between the second transmitting area and thefour first transmitting areas surrounding the second transmitting areas,whereby patterns corresponding to the first transmitting areas and thesecond transmitting area are separately formed, and the method comprisesan exposure step using the phase-shifting mask.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a phase-shifting mask according to a firstembodiment of the invention;

FIG. 2 is a graph for showing a special distribution of transmittedlight intensity obtained through simulation using the phase-shiftingmask of FIG. 1;

FIG. 3A is a plan view for showing an exposure pattern formed by usingthe phase-shifting mask of FIG. 1 obtained through simulation and FIG.3B is a plan layout diagram for schematically showing an exemplifiedrelationship between an active area and a contact hole in a memory cellarray of a DRAM manufactured by using the phase-shifting mask of FIG. 1;

FIG. 4 is a plan view of a phase-shifting mask according to a secondembodiment of the invention;

FIG. 5 is a graph for showing a special distribution of transmittedlight intensity obtained through simulation using the phase-shiftingmask of FIG. 4;

FIG. 6 is a plan view for showing an exposure pattern formed by usingthe phase-shifting mask of FIG. 4 obtained through simulation;

FIG. 7 is a plan view of a phase-shifting mask according to a thirdembodiment of the invention;

FIG. 8 is a graph for showing a special distribution of transmittedlight intensity obtained through simulation using the phase-shiftingmask of FIG. 7;

FIG. 9 is a plan view for showing an exposure pattern formed by usingthe phase-shifting mask of FIG. 7 obtained through simulation;

FIG. 10 is a plan view of a phase-shifting mask according to a fourthembodiment of the invention;

FIG. 11 is a sectional view of the phase-shifting mask of the fourthembodiment;

FIG. 12 is a plan view for showing an exposure pattern formed by amodification of the phase-shifting mask of FIG. 10 obtained throughsimulation;

FIG. 13 is a plan view for showing an exposure pattern formed by thephase-shifting mask of FIG. 10 obtained through simulation;

FIGS. 14A through 14F are sectional views for showing procedures in amethod of manufacturing an electronic device according to the invention;and

FIG. 15A is a plan view of an aperture used in on-axis illumination,FIG. 15B is a plan view of an aperture used in off-axis illumination,FIG. 15C is a plan view of a transfer pattern formed through exposure bythe on-axis illumination and FIG. 15D is a plan view of a transferpattern formed through exposure by the off-axis illumination.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings.

EMBODIMENT 1

FIG. 1 shows a phase-shifting mask according to a first embodiment ofthe invention. As is shown in FIG. 1, the phase-shifting mask includes atransparent photomask plate (with a thickness of 0.25 inch) of glass orthe like, an opaque portion 14 formed in the transparent photomask plateand transmitting portions formed in the transparent photomask plate. Theopaque portion is formed by providing an opaque film on the transparentphotomask plate. The opaque film is formed by patterning a metallic thinfilm of chromium (Cr) or the like. The transmitting portions formed asopenings in the opaque film includes plural first transmitting areas 11and 12 arranged periodically and two-dimensionally along a firstdirection (the X-axis direction) and a second direction (the Y-axisdirection). Each of the first transmitting areas 11 and 12 has asubstantially rectangular shape.

More specifically, the first transmitting areas 11 and 12 arealternately arranged with an equal pitch along the X-axis direction andthe Y-axis direction. In other words, the first transmitting areas 11and 12 are regularly and periodically arranged in an array formed byrows parallel to the X-axis direction and columns parallel to the Y-axisdirection.

A pitch between the first transmitting areas 11 measured along theX-axis direction is equal to a pitch between the first transmittingareas 12 measured along the X-axis direction. A pitch between theadjacent first transmitting area 11 and first transmitting area 12measured along the X-axis direction is equal to a pitch between theadjacent columns along the X-axis direction. On the other hand, a pitchbetween the first transmitting areas 11 measured along the Y-axisdirection is equal to a pitch between the first transmitting areas 12measured along the Y-axis direction. A pitch between the adjacent firsttransmitting area 11 and first transmitting area 12 arranged in theY-axis direction is equal to a pitch between the adjacent rows.

The mask is provided with a phase-shifting structure (phase shifters) sothat the phase of light transmitted by the first transmitting area 11can be different from the phase of light transmitted by the firsttransmitting area 12. In this embodiment, for the purpose of causing thephase difference, the main surface of the transparent photomask plate isprovided with steps by etching a part of the main surface, so that thesurface of the first transmitting area 11 can be positioned at adifferent level from the surface of the first transmitting area 12. Thisstructure will be described in detail below.

The transmitting portions further include second transmitting areas 13each formed in an area surrounded with the adjacent four firsttransmitting areas 11 and 12. The second transmitting areas 13 are alsoformed in areas corresponding to openings in the opaque film similarlyto the first transmitting areas 11 and 12. The surface of the secondtransmitting area 13 is positioned at a different level from thesurfaces of the first transmitting areas 11 and 12, so that a phasedifference can be caused between the second transmitting area 13 and thefirst transmitting areas 11 and 12. The differences in the levels of thesurfaces of the transmitting areas 11 through 13 are determined inconsideration of a wavelength of light used for exposure and arefractive index of the transparent photomask plate. The leveldifferences in the surfaces of the transmitting areas 11 through 13 arecaused not only by recessing the main surface of the photomask plate byetching or the like but also by depositing a phase-shifting thin film ona predetermined area on the main surface of the photomask plate. Whenthe level differences are caused in the surfaces of the transmittingareas 11 through 13, a difference is caused in an optical path ofexposure light when the transparent photomask plate transmits the light,resulting in causing light interference. This light interference plays asignificant role in the formation of a pattern by using thephase-shifting mask of this invention.

The plan layout of the transmitting areas 11, 12 and 13 is designed soas to form a pattern of contact holes in the exposure using thephase-shifting mask of this embodiment. Owing to the phase-shiftingstructure provided to the transparent photomask plate, a phasedifference of 90 degrees; is caused between the first transmitting area11 and the second transmitting area 13 and a phase difference of 90degrees is also caused between the first transmitting area 12 and thesecond transmitting area 13. Furthermore, a phase difference of 180degrees is caused between the first transmitting area 11 and the firsttransmitting area 12.

Each of the transmitting areas 11 through 13 of this embodiment isformed in a square with a side of 0.23 μm (hereinafter referred to as a0.23 μm square). The pitch between the first transmitting area 11 andthe first transmitting area 12 adjacent to each other along the X-axisdirection is set at 0.68 μm, and the pitch between the firsttransmitting area 11 and the first transmitting area 12 adjacent to eachother along the Y-axis direction is set at 0.34 μm. Each of the secondtransmitting areas 13 is formed at the center of an area surrounded withadjacent four first transmitting areas 11 and 12. In this case, thephase-shifting structure is designed so that a KrF laser with awavelength of 248 nm can be used as a source of the exposure light. Inthe phase-shifting mask of this embodiment, since ⅕ reduction projectionexposure is adopted, the dimensions of respective patterns on thephotomask plate are five times as large as the dimensions of patterns tobe transferred onto a resist mask. Accordingly, the actual dimension ofeach of the transmitting areas 11 through 13 is 0.23 μm×5, but for thepurpose of avoiding confusion, the dimensions of the patterns on thephase-shifting mask are herein described by using the dimensions of thepatterns formed through the projection exposure.

FIG. 2 shows a spacial distribution of intensity of light transmitted bythe phase-shifting mask of FIG. 1 obtained by using a light intensitysimulator. In FIG. 2, a solid line indicates the intensity of lightobtained in a position corresponding to a line a-a′ in FIG. 1, and abroken line indicates the intensity of light obtained in a positioncorresponding to a line b-b′ in FIG. 1. As is obvious from FIG. 2, alight intensity peak with a strong contrast appears in the firsttransmitting areas 11 and 12 according to the principle of thealternating phase-shifting mask. Also, it is confirmed that a lightintensity peak with a comparatively strong contrast appears also in thesecond transmitting area 13.

FIG. 3A shows an exposure pattern obtained through exposure of anegative resist by using the phase-shifting mask of FIG. 1. In thisexposure, a threshold intensity is set at 0.35. As is shown in FIG. 3A,openings in the resist can be formed not only in positions regularlyarranged in an array of rows and columns but also in shifted positions.FIG. 3B is a plan layout diagram for schematically showing anexemplified relationship between an active area of a memory cell arrayand a contact hole in a DRAM manufactured by using the phase-shiftingmask of FIG. 1. In each of active areas 110, two transistors are formed,and as is shown in FIG. 3B, three contact holes are disposed in eachactive area 110. A contact hole 111 is formed by light transmitted bythe first transmitting area 11 of FIG. 1, and a contact hole 112 isformed by light transmitted by the first transmitting area 12 of FIG. 1.These contact holes 111 and 112 are used for interconnecting a sourceand a storage electrode (not shown) in the active area 110. Also, acontact hole 113 is formed by light transmitted by the secondtransmitting area 13 of FIG. 1, and is used for interconnecting a drainand a bit line (not shown) positioned at the center of the active area110. The respective active areas 110 are separated from one another byan isolation 100.

In this manner, when the phase-shifting mask of this embodiment is used,an opening array pattern, in which openings each with a small diameterof approximately 0.2 μm are arranged with a small pitch of 0.4 μm orless, can be formed owing to the first transmitting areas regularlyarranged in an array of rows and columns for attaining an alternatingphase-shifting structure and the second transmitting area formed in anarea surrounded with the first transmitting areas and having a phasedifference of 90 degrees from the surrounding first transmitting areas.

Herein, “a phase difference of 180 degrees” means a phase difference of360°×N±180°, and “a phase difference of 90 degrees” means a phasedifference of 360°×M±90° (wherein N and M are arbitrary integers). Also,“a phase difference of substantially 180 degrees” widely includes aphase difference at which a pattern that cannot be resolved without thephase difference can be resolved according to the principle of analternating phase-shifting mask. Furthermore, “a phase difference ofsubstantially 90 degrees” widely includes a phase difference at whichlight transmitted by the aforementioned second transmitting area canform a pattern. According to a result of simulation, a phase differencebetween the adjacent two first transmitting areas is preferably in arange between 160 degrees and 200 degrees. Also, a phase differencebetween the second transmitting area and the surrounding firsttransmitting areas is preferably in a range between 70 degrees and 110degrees.

EMBODIMENT 2

A phase-shifting mask according to a second embodiment of the inventionwill now be described with reference to FIGS. 4 through 6.

The phase-shifting mask of this embodiment shown in FIG. 4 includes,similarly to the phase-shifting mask of FIG. 1, a transparent photomaskplate, an opaque portion 24 formed in the transparent photomask plateand transmitting portions formed in the transparent photomask plate. Thetransmitting portions formed as openings in an opaque film used as theopaque portion include plural first transmitting areas 21 and 22arranged periodically and two-dimensionally along a first direction (theX-axis direction) and a second direction (the Y-axis direction). Each ofthe first transmitting areas 21 and 22 has a substantially rectangularshape.

The dimensions and locations of the first transmitting areas 21 and 22are the same as those of the first transmitting areas 11 and 12 of thephase-shifting mask of the first embodiment shown in FIG. 1. Differentlyfrom the first embodiment, as is obvious from comparison between FIGS. 4and 1, each of second transmitting areas 23 has a dimension (0.27 μmsquare) larger than the dimension of the first transmitting areas 21 and22 (0.23 μm square) in this embodiment.

FIG. 5 shows an intensity spacial distribution of light transmitted bythe phase-shifting mask of FIG. 4 obtained by using a light intensitysimulator. In FIG. 5, a solid line indicates light intensity obtained ina position corresponding to a line c-c′ in FIG. 4, and a broken lineindicates light intensity obtained in a position corresponding to a lined-d′ in FIG. 4. As is obvious from FIG. 5, a light intensity peak with astrong contrast appears in the first transmitting areas 21 and 22 inaccordance with the principle of an alternating phase-shifting mask.Also, it is confirmed that a light intensity peak with a comparativelystrong contrast appears in the second transmitting area 23. Since thedimension of the second transmitting area 23 is thus increased in thisembodiment, the value of the light intensity peak appearing in thesecond transmitting area 23 can be increased to be substantially thesame as the value of the light intensity peak appearing in the firsttransmitting areas 21 and 22.

FIG. 6 shows an exposure pattern obtained through exposure of a negativeresist by using the phase-shifting mask of FIG. 4. In this exposure, athreshold intensity is set at 0.35. Thus, openings in the resist can beformed in the positions regularly arranged in an array of rows andcolumns as well as openings substantially the same size can be formed inshifted positions.

In this manner, when the phase-shifting mask of this embodiment is used,an opening pattern, in which openings each having substantially the samesize with a small diameter of approximately 0.2 μm are arranged with asmall pitch of 0.4 μm or less, can be formed owing to the firsttransmitting areas regularly arranged in an array of rows and columnsfor attaining an alternating phase-shifting structure and the secondtransmitting area formed in an area surrounded with the firsttransmitting areas and having a phase difference of 90 degrees from thesurrounding first transmitting areas.

EMBODIMENT 3

A phase-shifting mask according to a third embodiment of the inventionwill now be described with reference to FIGS. 7 through 9.

The phase-shifting mask of this embodiment shown in FIG. 7 includes,similarly to the phase-shifting mask of FIG. 4, a transparent photomaskplate, an opaque portion 34 formed in the transparent photomask plateand transmitting portions formed in the transparent photomask plate. Thetransmitting portions formed as openings in an opaque film used as theopaque portion include plural first transmitting areas 31 and 32arranged periodically and two-dimensionally along a first direction (theX-axis direction) and a second direction (the Y-axis direction). Each ofthe first transmitting areas 31 and 32 has a substantially rectangularshape.

The locations of the first transmitting areas 31 and 32 are the same asthose of the first transmitting areas 21 and 22 of the phase-shiftingmask of the second embodiment shown in FIG. 4. Differently from thesecond embodiment, as is obvious from comparison between FIG. 7 and FIG.4, the shape and the dimension of the first transmitting areas 31 and 32of this embodiment are different from those of the first transmittingareas 21 and 22 of the second embodiment. More specifically, thedimension of each of the first transmitting areas 31 and 32 measuredalong the Y-axis direction (i.e., 0.22 μm) is larger than the dimensionthereof measured along the X-axis direction (i.e., 0.19 μm) in thisembodiment.

FIG. 8 shows an intensity spacial distribution of light transmitted bythe phase-shifting mask of FIG. 7 obtained by using a light intensitysimulator. In FIG. 8, a solid line indicates the light intensityobtained in a position corresponding to a line e-e′ of FIG. 7, and abroken line indicates the light intensity obtained in a positioncorresponding to a line f-f′ of FIG. 7. As is obvious from FIG. 8, alight intensity peak having a strong contrast appears in the firsttransmitting areas 31 and 32 according to the principle of analternating phase-shifting mask.

Since the first transmitting areas 31 and 32 are arranged with a smallerpitch along the Y-axis direction, the interference between lighttransmitted by the adjacent transmitting areas along the Y-axisdirection is enhanced. Accordingly, while the exposure pattern iselongated in the X-axis direction in FIGS. 3A and 6, a comparativelyround exposure pattern can be obtained in this embodiment because thedimension of each of the first transmitting areas 31 and 32 along theY-axis direction is larger than that along the X-axis direction.

FIG. 9 shows an exposure pattern obtained through exposure of a negativeresist by using the phase-shifting mask of FIG. 7. In this exposure, athreshold intensity is set at 0.35. In this manner, round openingsarranged regularly in an array of rows and columns can be formed as wellas openings substantially the same size can be formed in shiftedpositions.

In this manner, when the phase-shifting mask of this embodiment is used,an opening pattern, in which round openings each having substantiallythe same size with a small diameter of approximately 0.2 μm are arrangedwith a small pitch of 0.4 μm or less, can be formed owing to the firsttransmitting areas regularly arranged in an array of rows and columnsfor attaining an alternating phase-shifting structure and the secondtransmitting area formed in an area surrounded with the firsttransmitting areas and having a phase difference of 90 degrees from thesurrounding first transmitting areas.

EMBODIMENT 4

FIG. 10 is a plan view for showing a phase-shifting mask according to afourth embodiment of the invention. The phase-shifting mask of FIG. 10includes, similarly to the phase-shifting mask of FIG. 7, a transparentphotomask plate, an opaque portion 44 formed in the transparentphotomask plate and transmitting portions formed in the transparentphotomask plate. The transmitting portions formed as openings in anopaque film used as the opaque portion include transmitting areas 41, 42and 43 and the plan layout of these transmitting areas 41, 42 and 43 isthe same as that of the transmitting areas 31, 32 and 33 of the thirdembodiment.

FIG. 11 is a sectional view of the phase-shifting mask taken on lineg-g′ of FIG. 10. As is shown in FIG. 11, the main surface of thetransparent photomask plate 45 is partially covered with a shadingmaterial film 46 of a Cr film or the like. On the main surface of thetransparent photomask plate 45, portions 47, 48 and 49 not covered withthe opaque material film 46 respectively correspond to the firsttransmitting areas 41 and 42 and the second transmitting area 43.

A recess type phase-shifting structure is formed by recessing thetransparent photomask plate 45 through etching or the like from its mainsurface to a given depth. Through such a recess, the thickness of thetransparent photomask plate 45 is varied in the transmitting areas,thereby causing a difference in the optical path. As a result, a phasedifference is caused in light transmitted by the respective transmittingareas.

When a wavelength of exposure light is indicated as λ, the refractiveindex of the photomask plate 45 is indicated as n and the depth of arecess in the photomask plate 45 is indicated as d, a phase difference xis given as x=360/d (n−1). In this embodiment, the portion 49 in thephotomask plate 45 corresponding to the second transmitting area 43 isnot recessed with the portion 47 corresponding to the first transmittingarea 41 recessed to a depth of λ(n−1)/4, and with the portion 48corresponding to the first transmitting area 42 recessed to a depth of3λ(n−1)/4. In this manner, a phase difference of 90 degrees is causedbetween the portion 47 and. the portion 49 and between the portion 48and the portion 49. and a phase difference of 180 degrees is causedbetween the portion 47 and the portion 48.

In the recess type phase-shifting structure, micro-roughness can beformed on the etched surface resulting from the etching of the photomaskplate 45, so that the light transmittance in the rough surface portioncan be degraded. In this embodiment, however, the second transmittingarea 43 is not etched, and hence, no micro-roughness is caused in thesecond transmitting area 43. Therefore, the light transmittance in thesecond transmitting area 43 can be prevented from being degraded ascompared with that of the other areas. When the second transmitting area43 is formed by etching the main surface of the photomask plate 45, thedegradation in the light transmittance caused in the second transmittingarea 43 can largely affect the formation of the exposure pattern.

FIG. 12 shows an exposure pattern obtained by using a phase-shiftingmask in which the portions 48 and 49 respectively corresponding to thefirst transmitting areas 41 and the second transmitting area 43 arerecessed with the portion 47 corresponding to the first transmittingarea 42 not recessed. This exposure pattern is obtained by using a lightintensity simulator. In this simulation, a threshold intensity in theexposure is set at 0.35, the light transmittance of the portion 48 ofthe photomask plate 45 is set at 0.8 and the light transmittance of theportions 47 and 49 is set at 1. The plan layout of these transmittingareas is the same as that of the fourth embodiment.

As is obvious from FIG. 12, openings formed correspondingly to the firsttransmitting areas 41 and 42 have different dimensions, and an openingcorresponding to the second transmitting area 43 is not formed. Incontrast, when the phase-shifting mask of the fourth embodiment is used,openings formed correspondingly to the first transmitting areas 41 and42 have the same dimension as is shown in FIG. 13. In simulation byusing the phase-shifting mask of the fourth embodiment, the lighttransmittance of the portions 47 and 48 is set at 0.8 and the lighttransmittance of the portion 49 is set at 1.

In this manner, when the phase-shifting mask of this embodiment is used,an opening pattern, in which round openings each having substantiallythe same size with a small diameter of approximately 0.2 μm are arrangedwith a small pitch of 0.4 μm or less, can be formed at high accuracyowing to the first transmitting areas formed by recessing and regularlyarranged in an array of rows and columns for attaining an alternatingphase-shifting structure and the second transmitting area formed withoutrecessing in an area surrounded with the first transmitting areas andhaving a phase difference of 90 degrees from the surrounding firsttransmitting areas.

EMBODIMENT 5

A method of manufacturing an electronic device according to a fifthembodiment of the invention will now be described with reference toFIGS. 14A through 14F. In this embodiment, a DRAM including refinedmemory cells is exemplified as the electronic device.

First, after forming an isolation region 51 in a semiconductor substrate50 by a known manufacturing method for a semiconductor integratedcircuit device as is shown in FIG. 14A, word lines of MOS transistorsare formed in the p-type semiconductor substrate 50. Each word lineincludes, for example, a polysilicon film 52 and a tungsten silicidefilm 54 that are capped with a nitride film 53. After depositing anitride film on the substrate 50 so as to cover the word lines, thenitride film is etched back from its surface, thereby forming a sidewallspacer of the nitride film at the side of the word line. Subsequently,for example, phosphorus (P) ions are implanted at 2×10¹³ cm⁻² into thesubstrate 50 at an acceleration energy of 10 keV, thereby forming a lowconcentration n-type diffusion region 55 in the substrate 50.

Next, as is shown in FIG. 14B, after forming a first interlevelinsulating film 56 covering the transistor structure, a photoresist 57is patterned by the lithography using a phase-shifting mask of thepresent invention. At this point, the transmitting areas 11, 12 and 13shown in FIG. 1 respectively define openings 58′, 59′ and 60′ of theresist 57. In FIG. 14B, merely three openings 58′, 59′ and 60′ are shownbut a large number of openings are actually formed as is shown in theplan layout of FIG. 3. The openings 58′ and 59′ correspond to the firsttransmitting areas of the phase-shifting mask, and the opening 60′corresponds to the second transmitting area. Each of the openings 58′and 59′ formed in this embodiment has a dimension of 0.24 μm, theopening 60′ has; a dimension of 0.28 μm, and the minimum pitch betweenthe openings 58′ through 60′ is 0.38 μm.

Next, as is shown in FIG. 14C, holes are formed in the first interlevelinsulating film 56 by using the photoresist 57 as an etching mask, andthe holes are then buried with an n-type polysilicon film. Then, thesurfaces of the n-type polysilicon film and the interlevel insulatingfilm 56 are planarized by CMP (chemical mechanical polishing), andstorage node contact holes are buried with plugs 58 and 59 and a bitline contact hole is buried with a plug 60. The plugs 58 through 60 areformed out of the n-type polysilicon film.

Then, as is shown in FIG. 14D, after an insulating film 61 is depositedon the first interlevel insulating film 56, a contact hole for the plug60 is formed in the insulating film 61. After a tungsten film isdeposited on the insulating film 61, the tungsten film is patterned,thereby forming a bit line 62. The bit line 62 is in contact with theplug 60 through the opening of the insulating film 61.

Subsequently, as is shown in FIG. 14E, a second interlevel insulatingfilm 63 is deposited on the insulating film 61 so as to cover the bitline 62, and then, contact holes for the plugs 58 and 59 are formed inthe second interlevel insulating film 63. Then, after these contactholes are buried with an n-type polysilicon film, the surface isplanarized by the CMP, and a storage node contact 64 is formed.

Next, as is shown in FIG. 14F, a storage electrode 65, a capacitanceinsulating film 66 and a plate electrode 67 are formed on the secondinterlevel insulating film 63. The storage electrode 65 is electricallyconnected with the plug 58 or 59 in the first interlevel insulating filmthrough the storage node contact 64.

According to this manufacturing method, the storage node contact holescan be formed with a small pitch of 0.4 μm or less, and the bit linecontact holes can be simultaneously formed in positions shifted from thestorage node contact holes by a ½ pitch. As a result, one refined memorycell can be formed in an area smaller than 0.3 μm², thereby realizingthe integration of a 1 Gbit DRAM.

In the exposure conducted in each of the aforementioned embodiments, anaperture in the shape as is shown in FIG. 15A is used. Numerals shown inFIG. 15A correspond to the ratios of the position and the size of anopening to a radius of a disk-shaped opaque portion of the aperture. Instead of using the aperture as is shown in FIG. 15A, an aperture in theshape as is shown in FIG. 15B can be used. Illumination conducted byusing the aperture of FIG. 15A can be designated as on-axisillumination, and illumination conducted by using the aperture of FIG.15B can be designated as off-axis illumination. The off-axisillumination can include another form. The aperture of FIG. 15B has fouropenings. In the on-axis illumination, when a focus error is causedduring the exposure, the transfer pattern can be distorted as is shownin FIG. 15C. However, in the off-axis illumination, even when a focuserror is caused during the exposure, a well-shaped transfer pattern withless distortion can be obtained as is shown in FIG. 15D. The transferpatterns of FIGS. 15C and 15D are obtained through simulation.

The phase-shifting mask of this invention is widely applicable to alayout including a large number of isolated patterns regularly arrangedin an array of rows and columns and another isolated pattern disposed ina position shifted from the former isolated patterns. Specifically, eachof the transmitting areas is not required to correspond to an openingfor a contact hole or the like. For example, a light transmitting areacan correspond to a fine structure such as a quantum dot.

Also, the phase-shifting mask of this invention is applicable to a planlayout in which at least four isolated patterns and another patternsurrounded with the four isolated patterns are disposed close to oneanother.

Furthermore, the phase-shifting mask of this invention can be used inanother process than the exposure of a resist. For example, in a thinfilm depositing process or an etching process, a kind of exposure can beconducted by using the phase-shifting mask of this invention, therebyproviding the thin film depositing or etching process with an opticaleffect. A degree of the influence of such an optical effect on theprocess is varied in accordance with a light intensity spacialdistribution on a plane, and hence, a fine structure can be thus formed.

In the phase-shifting mask of this invention, adjacent two firsttransmitting areas mutually cause a phase difference of substantially180 degrees in exposure light and a second transmitting area andsurrounding four first transmitting areas cause a phase difference ofsubstantially 90 degrees in the exposure light. Accordingly, isolatedpatterns regularly arranged in an array of rows and columns and anisolated pattern disposed in positions shifted from the former isolatedpatterns can be simultaneously formed through the exposure.

In this manner, by providing the phase difference of 90 degrees, apattern with a high resolution can be transferred onto an irregularposition close to other patterns, in which a pattern cannot be formed byusing a conventional alternating phase-shifting mask. This can be veryeffective in the formation of a contact hole pattern includingirregularly arranged holes at a high density in, particularly, asemiconductor integrated circuit device.

What is claimed is:
 1. A phase-shifting mask comprising a transparentphotomask plate, an opaque portion formed of an opaque film on saidtransparent photomask plate and transmitting portions formed of openingsin said opaque film, wherein said transmitting portions include: pluralfirst transmitting areas periodically arranged with said opaque portiondisposed therebetween, said first transmitting areas being arrangedalong a first direction and a second direction different from said firstdirection; and at least one second transmitting area provided in acentral portion of an area surrounded with four first transmitting areaswhich are adjacent to each other along said first and second directionsamong said plural first transmitting areas, said second transmittingarea and each of said four first transmitting areas surrounding saidsecond transmitting area being arranged with said opaque portiondisposed therebetween, a phase difference of substantially 180 degreesin exposure light is caused between two first transmitting areas whichare adjacent to each other along said first direction or said seconddirection among said plural first transmitting areas, a phase differenceof substantially 90 degrees in said exposure light is caused betweensaid second transmitting area and said four first transmitting areassurrounding said second transmitting area.
 2. The phase-shifting mask ofclaim 1, wherein a surface of at least a part of said plural firsttransmitting areas is positioned at a different level from a mainsurface of said transparent photomask plate.
 3. The phase-shifting maskof claim 2, wherein a surface of at least a part of said plural firsttransmitting areas is recessed to be lower than the main surface of saidtransparent photomask plate.
 4. The phase-shifting mask of claim 2 or 3,wherein a surface of said second transmitting area is positioned at adifferent level from surfaces of said plural first transmitting areas.5. The phase-shifting mask of claim 4, wherein the surface of saidsecond transmitting area is positioned at the same level as the mainsurface of said transparent photomask plate.
 6. The phase-shifting maskof claim 1, wherein the phase difference caused between two firsttransmitting areas which are adjacent to each other along the first orsecond direction among said plural first transmitting areas is in arange between 160 degrees and 200 degrees.
 7. The phase-shifting mask ofclaim 1 or 6, wherein the phase difference caused between said secondtransmitting area and said four first transmitting areas surroundingsaid second transmitting area is in a range between 70 degrees and 110degrees.
 8. The phase-shifting mask of claim 1, wherein said firstdirection is perpendicular to said second direction.
 9. Thephase-shifting mask of claim 1, wherein each of said plural firsttransmitting areas has a shape for forming a hole.
 10. Thephase-shifting mask of claim 1, wherein said second transmitting areahas a dimension larger than a dimension of each of said firsttransmitting areas.
 11. The phase-shifting mask of claim 1, wherein apitch of said first transmitting areas measured along said firstdirection is approximately twice as large as a pitch of said firsttransmitting areas measured along said second direction.
 12. Thephase-shifting mask of claim 1, wherein a dimension of each of saidfirst transmitting areas measured along said second direction is smallerthan a dimension thereof measured along said first direction.
 13. Aphase-shifting mask comprising a transparent photomask plate, an opaqueportion formed of an opaque film on said transparent photomask plate andtransmitting portions formed of openings in said opaque film, whereinsaid transmitting portions include: at least four first transmittingareas arranged with said opaque portion disposed therebetween, saidfirst transmitting areas being arranged along a first direction and asecond direction different from said first direction; and at least onesecond transmitting area provided in a central portion of an areasurrounded with said four first transmitting areas, said secondtransmitting area and each of said four first transmitting areas beingarranged with said opaque portion disposed therebetween, a phasedifference of substantially 180 degrees in exposure light is causedbetween two first transmitting areas which are adjacent to each otheralong said first direction or said second direction among said fourfirst transmitting areas, and a phase difference of substantially 90degrees in said exposure light is caused between said secondtransmitting area and said four first transmitting areas surroundingsaid second transmitting areas, whereby patterns corresponding to saidfirst transmitting areas and said second transmitting area areseparately formed.